05 Trailblazer random misfire

4.2L engine, about 78,000 miles. Sometimes runs fine, sometime runs rough, stalls and throws codes usually misfire detected #2 cylinder, but sometimes random misfire detected. Fuel pressure at idle fluctuates rapidly between 58 and 65, smoths out at higher RPM. Replaced plugs, #2 coil, cleaned throttle body and MAF sensor. Continues. May run good for weeks and thousands of miles then acts up again. Been chasing this problem for 18 months. Any ideas?
 

OVERKILL

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Originally Posted By: Nick R
It's another coil. We had this issue with the expedition. We replaced 3 coils total over the course of a bout 2 years.
Did you ever replace the plugs on that? Reason I ask, is that I replaced two coils on ours, and then, when I replaced the plugs, found that the plugs that had lost their platinum "pucks" were the ones that needed the new coils..... grin OP: Could also be a partially plugged/sticking injector for that cylinder.
 

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Originally Posted By: another Todd
Yes, the plugs were the first thing I replaced.
Todd: The first part of my reply was in response to Nick's point about his Expedition and the coils. I saw from your first post that you had already done the plugs and a coil. Have you tried some injector cleaner?
 
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Originally Posted By: another Todd
Injector cleaner? Dozens of bottles...the good stuff too.
I'd recommend you do a compression test - especially on the cylinder that is misfiring. Some people have had problems with the valve seats giving way. Have you searched at Trailvoy.com? There are a lot of good people there and I remember this subject came up occasionally. Good luck.
 

JTK

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Originally Posted By: oldmaninsc
Have you searched at Trailvoy.com? There are a lot of good people there and I remember this subject came up occasionally. Good luck.
+1 to that for GMT 360 chassis info. Back when I owned my 2005 Trailblazer I spent a lot of time there.
 
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Originally Posted By: oldmaninsc
Originally Posted By: another Todd
Injector cleaner? Dozens of bottles...the good stuff too.
I'd recommend you do a compression test - especially on the cylinder that is misfiring. Some people have had problems with the valve seats giving way. Have you searched at Trailvoy.com? There are a lot of good people there and I remember this subject came up occasionally. Good luck.
+1 A former boss of mine worked a a GMC dealer when the Envoy and Trailblazer came out. He quickly became an expert in replacing those cylinder heads. You should replace the timing chain and hardware when you take the heads off.
 

another Todd

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Pulled all the coils today, everything looked OK, a couple cylinders had a little oil in the spark plug well, but I don't think enough to cause a problem. Looks like it is weeping through a porus casting. Plugs looked fine also. Put it back together with some grease on the boots and plugs and it is running perfect right now. We'll see. What about that fast fluctuating fuel presssure...58-65 at idle only.
 
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Originally Posted By: oldmaninsc
Originally Posted By: another Todd
Injector cleaner? Dozens of bottles...the good stuff too.
I'd recommend you do a compression test - especially on the cylinder that is misfiring. Some people have had problems with the valve seats giving way. Have you searched at Trailvoy.com? There are a lot of good people there and I remember this subject came up occasionally. Good luck.
I believe the valve seat problem was the reason for the head replacement-ck with a Chevy dealer for a possible special policy extendending the warranty on the head.
 

JTK

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Originally Posted By: another Todd
Pulled all the coils today, everything looked OK, a couple cylinders had a little oil in the spark plug well, but I don't think enough to cause a problem. Looks like it is weeping through a porus casting. Plugs looked fine also. Put it back together with some grease on the boots and plugs and it is running perfect right now. We'll see. What about that fast fluctuating fuel presssure...58-65 at idle only.
If I remember correctly, the 2005+ had the 'improved' coils. Previous years were the most problematic. Also, I believe the valve seat issue was on the earliest model 4.2L I6's. Head removal is a huge job given the mile long timing chain and broken bolts & pulled threads that will happen on aluminum block/head engine. Any oil in the plug wells would be due to some leakage through the plug-well seals. I forget if they are O-rings or RTV on these. No worries if it's just a little oil. 2005+ also included a MAF sensor, where earlier 4.2's used MAP sensors only. You could have issues with the MAF. They do make large enough fuel trim adjustments on these engines to cause random misfires. I recall some owners using oiled air filters on their Trailblazers, which gunked up their MAF's, leading to random misfires. The fuel system I believe was also changed in 2005, using a pulse-width modulated fuel pump (variable speed?) to control fuel pressure. No filter either, just a tank sock. Joel
 
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another Todd

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JTK, I was suspecting the variable speed fuel pump because my regulator looks different than what all the pars houses show. Mine has one fuel line going to it, and an electrical connector. I suppose this is what regulates the voltage to the pump.
 
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